There has recently been highlightened on a large scale an influence of particulate matter emitted from a diesel engine or the like or NOx upon environment. As an important means to remove such harmful matter, various studies are made with regard to usage of a honeycomb structure having porous partition walls as a filter, catalyst carrier, or the like.
For example, there has been developed a honeycomb structure for a filter for trapping particulate matter (hereinbelow referred to as PM) emitted from a diesel engine (the filter is hereinbelow referred to as DPF). A honeycomb structure for a DPF generally has porous partition walls disposed so as to form a plurality of through holes extending in an axial direction, and adjacent through holes are plugged at the opposite end portion. Exhaust gas is sent in the through holes opening at one end portion and passed through the partition walls inside the honeycomb structure, and thereby particulate matter in the exhaust gas can be trapped and removed.
Such a honeycomb structure is generally subjected to canning with metal when it is mounted on a car body and used in this state. Therefore, when the honeycomb structure has low isostatic strength, the honeycomb structure is sometimes damaged upon canning or during use in a condition that it is canned. Therefore, upon canning of the honeycomb structure, the honeycomb structure is required to have an isostatic strength of about IMPa or more. On the other hand, when a honeycomb structure is used as a DPF, a catalyst carrier, or the like, pressure loss of exhaust gas becomes a problem. That is, pressure loss of exhaust gas is caused because exhaust gas passes through a honeycomb structure, which leads to decrease in output or increase in fuel consumption of an internal combustion engine such as a diesel engine or a gasoline engine. With regard to this, it is effective to raise porosity or open frontal area of a honeycomb structure so as to decrease pressure loss. However, raising porosity or open frontal area of a honeycomb structure generally causes deterioration in isostatic strength of the honeycomb structure, and the honeycomb structure is prone to be damaged. Therefore, there has been desired a honeycomb structure which has high isostatic strength and is hardly damaged even if porosity or open frontal area of the honeycomb structure is raised so as to decrease pressure loss.
There is disclosed a honeycomb structure containing cordierite as the main component and having a thermal expansion coefficient of 3×10−6 or less, a porosity of 55 to 80%, and an average pore diameter of 25 to 40 μm for the purpose of providing a honeycomb structure having characteristics of high trapping, small pressure loss, and low thermal expansion coefficient (JP-A-9-77573). In addition, there is disclosed a honeycomb structure having a porosity of 55 to 80%, an average pore diameter of 30 to 50 μm, and Y/X≦0.05 (X represents capacity of the whole pores, and Y represents capacity of the pores each having a diameter of 100 μm or more.) for the purpose of providing an exhaust gas purification filter which is inhibited from being damaged or melted and which has excellent ability to remove particulate matter (JP-A-2002-357114). However, there has not been obtained a honeycomb structure which has small pressure loss and which is hardly damaged.